The present invention relates, in general, to semiconductor structures and, more particularly, to a semiconductor structure having a monocrystalline member overlying a cavity in a semiconductor substrate and a process therefor.
Sensor devices, and in particular accelerometers, often use a microstructure beam or other element as a part of the sensor. In the case of an accelerometer, the microstructure will include a movable beam or other mass disposed on a semiconductor chip that moves in response to external acceleration forces. This movement is converted by electronics either on the chip or on a related control die into electrical signals corresponding to the magnitude and direction of the acceleration. The sensor chip is typically mounted to a printed circuit board (PCB), such as with surface mount technology.
Some prior accelerometers measure acceleration in a direction perpendicular to the chip's surface (i.e. a z-axis). However, for some acceleration measurement applications, such as lateral acceleration measurement for air bag deployment in an automobile, such z-axis sensor chips must be mounted vertically relative to the PCB using a bracket or the like. This type of mounting, however, has several disadvantages. First, the bracket increases the degree of PCB-to-PCB spacing required and is expensive to provide with the chip. Also, such a sensor chip suffers from low sensitivity (i.e. is not suitable for low-g acceleration applications) and from a fairly low signal-to-noise ratio.
Further, such prior accelerometers use a capacitive plate that is parallel to the chip's surface. Thus, when the surface area of the plate is increased, such as to increase sensitivity, there is a corresponding, undesirable increase in parasitic capacitance between the plate and the chip's surface. Also, such increased surface area requires more of the chip's available surface area.
Another disadvantage of some current accelerometers is the use of polysilicon in the sensing elements. Polysilicon is susceptible to cracking and residual stresses that can negatively impact the performance of the sensor.
Accordingly, it would be desirable to have an accelerometer able to measure lateral acceleration without the need for vertical mounting on a PCB using a bracket and also suitable for high or low-g applications with an improved signal-to-noise ratio. It also would be desirable to have an accelerometer formed using materials more stable than polysilicon and to be able to increase the size of a capacitive plate in an accelerometer to improve sensitivity without significantly increasing parasitic capacitance .